This invention relates to a multi-component system and method for deep well mapping of zones of anomalous electrical resistivity. More particularly, the invention utilizes currently employed coated, corrosion resistant well casings to carry an electric current down to an exposed end of the production tubing portion at depth whereby the strength of the transmitted current is sufficient to generate electric fields. The effect on these fields in anomalous zones proximate the end of the well is a function of the nature of the materials in the zone, which effects can be monitored by appropriately placed sensors.
Electric and electromagnetic geophysical methods are used to map the distribution of electrical resistivity in the subsurface of the earth. Generally known methods employ transmitters that induce electrical currents to flow in the ground. The transmitters can be sources of electric current injected by electrodes implanted in the soil or rock and connected to a power supply or the transmitters can be loops of wire carrying an alternating current which produces an alternating magnetic field that, by Faraday's law of induction, induces an electromotive force in the ground that, in turn, drives currents in the ground. In either case, the currents induced depend on the distribution of resistivity in the ground and these induced currents produce secondary electric and magnetic fields that can be measured by receivers which are usually separated from the transmitter. For instance, the receivers may include two separated electrodes in contact with the ground and across which a voltage is measured that is proportional to the electric field at that point. Receivers may also include a variety of sensors designed to measure the magnetic fields that accompany the induced currents. The transmitters and receivers can be on the surface or in the ground.
These methods can be used to determine the distribution of electrical resistivity in the ground. For example, the methods can be used to characterize the layering of the ground so as to identify a resistive layer that contains oil or gas, a conductive layer containing saline water, or a clay layer that might be an impermeable barrier for hot water in a geothermal setting, or the like. A more specific application of such methods is to determine the size and electrical resistivity of limited regions in the ground. Examples are zones of petroleum rich rock in an oilfield that has not been drained by the existing oil wells in the field (essentially bypassed oil), zones of electrically conducting rocks reflecting the presence of metallic ore minerals, a zone of enhanced conductivity brought about by the injection under pressure of a fluid mixture designed to cause a fracture or a fluid mixed with solid conductive particles intended to keep the fracture open (proppant), or a zone of decreased resistivity caused by the injection of carbon dioxide for sequestration for mapping and monitoring steam or chemicals injected to reduce viscosity and increase production from an oilfield formation. In all of these applications, the goal is to detect and, if possible, delineate a zone whose electrical resistivity is distinctly different from the resistivity of the overall volume of the ground below the surface in a specified region (referred to as the background resistivity).
A specific, known transmitter-receiver configuration that is particularly effective for detecting and delineating finite zones with a resistivity different from the background resistivity is an electric current source in which at least one of the electrodes is located at depth in the vicinity of the target zone. This configuration is shown in FIG. 1. This traditional surface-based configuration employs two current electrodes on the surface, A-B, that inject current I into the ground. Collectively this structure is usually referred to as the transmitter. Current is carried to each electrode from a power supply (not shown) by an insulated cable. Another pair of two separated electrodes on the surface, usually referred to as a dipole, is used to measure the voltage drop V between two points caused by the injected currents. This measuring dipole is usually referred to as the receiver. The measurement is usually described in terms of an electric field, in volts per meter, obtained by simply dividing the measured voltage by the separation distance L of the electrodes. The receiver dipole usually occupies successive positions on the surface over the target zone. A variant on this configuration, to which this invention is directed, uses a deep electrode B′ to inject the current adjacent to the anomalous zone being investigated.
In either case, the current in the ground is distorted by the presence of the anomalous zone. In the situation where the anomalous zone is less resistive than the surroundings, current is deflected or channeled into the zone and the resulting secondary fields, seen at some distance away such as in a nearby borehole or on the surface, can be represented by an induced current dipole in the zone whose strength is proportional to the size of the anomalous zone and the difference in resistivity. The electric fields measured along the surface are perturbed or offset from the value they would have in the absence of the anomalous zone. The fields on the surface are thus composed of the fields that would be present for the background in the absence of the anomalous zone, plus the secondary or anomalous fields caused by anomalous currents caused by the zone of anomalous resistivity. The measurement of the anomalous fields on the surface permits the determination of the depth, size and resistivity contrast of the particular target zone.
The perturbation in surface electric fields caused by a small zone is itself small and difficult to recognize in practical field data because normal surface field variations due to inhomogeneities in the background, and particularly due to near surface resistivity inhomogeneities, dwarf the anomalies of deep features. However, the goal of many electrical surveys is to detect changes in the zone of interest over time scales appropriate to the subsurface activity. The background resistivity can be assumed invariant over these scales and so small changes in resistivity in small zones at depth can be detected.
The importance of placing an electrode at the depth of the anomalous zone is shown quantitatively in FIG. 2. The target zone for this illustrative model is a vertical, 100 meter by 200 meter conductive sheet 202 which is oriented in the vertical or x-z plane, while both the transmitter and receiver are on the x axis. The sheet is characterized by the product of its conductivity and thickness (in this model the conductivity thickness product is 10 (note: conductivity is the reciprocal or inverse of resistivity and the units can be Siemens, S, per meter; the conductivity thickness is therefore in Siemens). The background resistivity in this example is 100 Ohm meters (ρ=100 Ωm). The secondary surface electric fields, Ex, are plotted as a function of distance from the well in FIG. 2B for two source current configurations: a surface bipole A-B, and an inverted L-shaped array A′-B′ with the B′ current electrode in the vicinity of the target.
The surface field anomaly from this deeply buried conductive zone is 100 times larger than that produced from the surface array when one of the electrodes is buried. The results in this figure are presented for surface electric fields in Volts per meter (V/m) for a source current of one Ampere (A). In a typical survey, a current of 10 A would be used and the voltage difference between two measuring points 100 m apart would be measured. The voltage measured in the model study shown would therefore be 1000 times the field values in the plot. For example, at a distance of 300 m from the well-head using the deep electrode, the secondary field is 10−8 Volts per meter so the voltage difference on a receiving dipole would be 10−5 Volts and easily measured. With this in mind, it becomes clear that a whole new window on subsurface features is opened if the current source can be located close to the desired zone of investigation.
The problem that has kept this deep source configuration from being implemented is that there has been no practical method of placing a current electrode with its attendant insulated current cable at the bottom of a typical drilled well. Almost all wells drilled for hydrocarbons, geothermal fluids or steam, carbon dioxide injection, water etc. are lined with a metallic pipe called a casing. A normal casing plan for a well involves successive casings of varying lengths and progressively smaller diameter. A large diameter hole is drilled through the near surface, usually unconsolidated, formations. A reduced diameter hole is then drilled and cased to greater depth and, finally, an even smaller diameter hole is drilled and cased to the desired maximum depth. The last, smallest diameter casing is referred to as the production casing. In some situations, a continuous length of tubing, referred to as the production tubing, is inserted inside the production casing. At each stage, cement is forced into the annular space between the drilled hole and the casing, with the cement filling the space for a short distance between the casings from the bottom of the larger casing. The space between the innermost casing and the next largest diameter casing in the upper portion of the well is empty. Most important, there needs to be continuous free access to the production casing. However, equipment devices called packers, which seal off certain depths in the well and tubing used to withdraw or inject fluids, occupy the production casing such that there is no room for a heavy current carrying cable or electrode.
With the above in mind, there is seen to be a need for a system and method which will enable a current electrode to be effectively provided at the bottom of a drilled well in order to enhance the ability to map subsurface zones of anomalous electrical resistivity.